P439: RNA SEQUESTRATION IN P-BODIES PROMOTES LEUKEMIA ONSET AND PROGRESSION
Authors: Ludovica Proietti, Srikanth Kodali, Gemma Valcarcel, Anna Vanessa Lopez-Rubio, Patrizia Pessina, Thomas Eder, Yingzhi Cui, Florencia Levin, Bernhard Alber, Gabriele Manhart, Alessandro Fatica, Jose Louis Sardina, Bruno DI Stefano, Florian Grebien
Journal: HemaSphere (2023)

Background: Leukemic transformation requires the subversion of the gene expression programs maintaining hematopoietic progenitor cell identity. A large body of work has established the role of transcriptional and epigenetic dysregulation in initiating and sustaining leukemia. However, the interplay between post-transcriptional and epigenetic regulatory mechanisms in leukemic cell self-renewal and its significance as a targetable vulnerability in AML remains poorly understood. The subcellular localization of RNA molecules influences their post-transcriptional fates. P-bodies are cytoplasmic biomolecular condensates involved in mRNA sequestration from the ribosomes to ensure their translational repression. Recent evidence suggests that core components of P-bodies are aberrantly expressed in various cancers, indicating that RNA sequestration could be sufficient to destabilize cell identity and sustain oncogenesis across tissue types. Yet, the functional, molecular and cellular roles of P-bodies in haematological malignancies and other cancers remain unexplored. Aims: We aimed to elucidate the role of post-transcriptional mechanisms in leukemia initiation and maintenance. Methods: We performed unbiased genome-wide CRISPR/Cas9 screens in normal and malignant hematopoietic progenitors to identify leukemia-specific dependencies. We extensively validated P-body requirements in AML using a variety of approaches such as polysome profiling, RNA immunoprecipitation, P-body isolation, RNA sequencing and proteomics as well as in vivo models. Results: Unbiased genome-wide CRISPR/Cas9 screens in AML vs. normal cells identified P-body regulators as specific dependencies in myeloid leukemia. We found that P-body assembly is dysregulated in AML and that P-bodies are functionally required for the self-renewal of AML cells but dispensable for healthy HSPCs. Also, P-body-mediated RNA sequestration was required for the initiation and maintenance of AML in vivo. We found that mRNAs that are sequestered in P-bodies are translationally repressed and enriched for transcripts encoding transcriptional and epigenetic regulators. In line with this, P-body dissolution caused upregulated protein expression of factors with potent tumour suppressive activity, leading to transcriptional changes associated with myeloid differentiation and rewiring of chromatin architecture in AML cells. Summary/Conclusion: We discovered that AML cells hijack P-body function to sequester and translationally repress mRNAs encoding tumor suppressive chromatin regulators, thereby promoting the establishment of a pro-leukemic chromatin architecture and transcriptional program. Our findings establish a molecular link in AML cells between RNA sequestration in cytoplasmic condensates and chromatin remodelling in the nucleus that can be exploited to design new precision therapies for AML and other malignancies. Keywords: Leukemogenesis, Myeloid leukemia, Acute myeloid leukemia, Gene regulation